leetcode 22-Generate Parentheses(medium)

Given n pairs of parentheses, write a function to generate all combinations of well-formed parentheses.

 

backtracking

a function to achieve backtracking: 

a list to record all the combination, a string str to record the trial subset at the moment, a number index to record the number of char in the subset, and a number to record left parentheses number(to make sure always on the right way to a valid combination)

1. index=n*2 means we have got an answer, add this subset to list;

2. left bracket number<n, means we can still add another left bracket to the subset;

3. right bracket number<left bracket, means we can still add another right bracket to the subset;

follow this rule to do backtracking.

class Solution {
    public List<String> generateParenthesis(int n) {
        List<String> list=new ArrayList<>();
        if(n==0) return list;
        generate(list, "", n, 0, 0);
        return list;
    }
    public void generate(List<String> list, String str, int n, int nleft, int index){
        if(index>=2*n){
            list.add(str);return;
        }
        if(nleft<n){
            generate(list, str+'(', n, nleft+1, index+1);
        }
        if(index-nleft<nleft){
            generate(list, str+')', n, nleft, index+1);
        }
    }
}